Studies are being conducted to correlate the role of biotransformation and covalent binding with the potential carcinogenicity of various aliphatic organohalogens. Compounds being tested at present include two known chemical carcinogens, trichloroethylene (TCE) and carbon tetrachloride (CCl4), a widely used gaseous anesthetic, halothane, and methylene chloride, currently replacing TCE as a solvent. The general approach is to incubate 14C-labeled organohalogens with isolated rat hepatocytes or microsomes plus calf thymus DNA and then to isolate and purify lipid, protein, DNA and RNA for quantitating the amount of bound 14C. In vivo experiments are also performed, usually by administrating the 14C-organohalogen intraperitoneally to rats. CCl4 and halothane show appreciable binding to hepatic proteins and lipids, with the highest degree of binding in the endoplasmic reticulum. Both compounds also bind to nuclear proteins and lipids. Only CCl4 binds covalently to nucleic acids. Both compounds are bioactivated along reductive pathways, since O2 decreases covalent binding. TCE binds to liver protein, lipids and nucleic acids. It is bioactivated by an O2 dependent pathways present in the endoplasmic reticulum. Methylene chloride binds to hepatic lipid and protein, but its binding is 10- to 20-fold less than that of TCE. This binding may occur nonenzymatically, but pretreatment of rats with phenobarbital appears to enhance binding. Studies in the coming year will focus on selecting species and strain variation in the covalent binding of these organohalogens. Particular emphasis will be placed on those strains of mice that show high carcinogenic response to the organohalogens. A major undertaking will be to determine the actual binding species for each organohalogen and the target molecule in each tissue macromolecule (i.e., particular DNA base or amino acid).